The body-cam video suddenly turns shaky as an Ohio police officer fires his gun into a car, leaving an unarmed black motorist dead, shot in the head. In Wisconsin, slow-motion images show an armed man lobbing his gun over a fence, then falling backward, before a police officer fires a fatal shot into his chest. And in Minnesota, a police dashboard camera captures an officer abruptly firing seven shots at the driver of a car he has stopped, as the man’s girlfriend and her young daughter watch from inside the car.

Each of these videos held an essential role in a courtroom this month, as jurors tried to decide whether the officers had committed crimes. Yet none of the cases brought convictions; in Cincinnati, the trial of the Ohio police officer ended in a mistrial on Friday, the second time the case had been tried without jurors’ agreeing on a verdict.

Since the 2014 shooting death of Michael Brown by a police officer in Ferguson, Mo. — a shooting that was not captured on video — there has been a nationwide push for more video recording of the police. But, as the three cases this month show, videos by no means ensure convictions.

“It’s not the end-all, be-all,” said John T. Chisholm, the district attorney for Milwaukee County, who presented video from two officers’ body cameras in the case against Dominique Heaggan-Brown, a police officer who fatally shot Sylville K. Smith in August. On Wednesday, Mr. Heaggan-Brown was acquitted.

Without body-camera video from the shooting of Mr. Smith, Mr. Chisholm said in an interview, he would not have brought criminal charges against the officer who fired the fatal shot.

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An image from a police video showing Diamond Reynolds, Mr. Castile’s girlfriend, handcuffed in the back of a police car with her daughter after the shooting.CreditMinnesota Bureau of Criminal Apprehension

“It was absolutely central to this case,” he said. “But for the body cameras there would not have been the confluence of facts and law that would have merited a criminal charge.”

But jurors, watching these images playing on projection screens in courtrooms, have not come away with simple answers. In some trials, a single piece of body-camera footage has been used to illustrate competing viewpoints: In the Milwaukee case, slowed-down, frame-by-frame video was used to show that the suspect had no weapon when he was shot a second time. The same video, played at regular speed, revealed a scene that was swift, confusing and chaotic, a boost to the defense.

Some jurors in these cases have said that, videos aside, they had been swayed most of all by officers’ assertions that they feared for their lives. And in some cases, the videos themselves do not fully show operative moments, leaving jurors to fill in the blanks.

Bonita Schultz, a juror in the case against Officer Jeronimo Yanez in Minnesota this month, said the jury had struggled with what it could not see from the dashboard camera video. Defense lawyers suggested that Philando Castile, who was fatally shot by Officer Yanez, was reaching for a gun in the moments before he was shot, though Mr. Castile’s girlfriend, in the passenger seat, said he was not.

The dashboard camera video did not provide a close-up view of the front seat, which might have resolved the point. But in the end, Ms. Schultz said, she was convinced by the officer’s account.

“It looked like he was trying to control the situation but couldn’t,” she said of Officer Yanez in the dashboard camera footage, which she described as “90 percent of the evidence” in the case. “It didn’t show everything, but it showed enough.”

The 12 jurors were at first almost evenly divided on whether to convict, then moved to an 8-to-4 split against convicting, then 10 to 2, Ms. Schultz said. Finally, she said, on the fifth day of deliberations, having covered a wall in the jury room with elaborate notes, they cleared him.

Although prosecutions of officers remain rare despite increased attention to police shootings and treatment of black people in recent years, juries have convicted at least eight police officers charged with manslaughter or murder since April 2012, according to Philip M. Stinson, a professor at Bowling Green State University who tracks such cases. They include Peter Liang, the New York City officer who fatally wounded Akai Gurley on a stairwell, and Robert C. Bates, a volunteer deputy in Oklahoma who said he had meant to use his Taser but instead grabbed his gun and shot Eric C. Harris, killing him.

At the same time, more police departments have equipped officers with body cameras, and more than half of medium and large departments have adopted or are testing such cameras.

Some people had presumed that more videos would hand juries clear-cut answers to accusations of police misconduct.

The videos “have changed things,” said David A. Harris, a law professor at the University of Pittsburgh who studies police behavior, police regulation and racial profiling. “But it was a little naïve to think they’d solve the problem, or get us all the way home on the question of guilt or innocence.”

In some courtrooms, the prospect of video evidence may give people an expectation that a jury’s decision should be obvious.

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An image from the body camera of Officer Raymond M. Tensing showed him drawing his gun as he approached Samuel DuBose’s car. The case went to trial twice without jurors’ reaching a verdict.CreditHamilton County Prosecuting Attorney

“People think, ‘Oh, video — awesome,’” said Shannon McMurray, a Tulsa lawyer who defended a white officer, Betty Jo Shelby, charged with manslaughter after fatally shooting an unarmed black man, Terence Crutcher, as a video camera rolled. “But it’s really misleading, and it can make it worse than if you don’t have a camera.”

The Tulsa jury acquitted Officer Shelby, but the foreman eventually wrote a four-page letter, entered into the court record, that revealed how torn the jury had been. The foreman, who was not identified by name, wrote that many jurors “could never get comfortable with the concept of Betty Shelby being blameless for Mr. Crutcher’s death.”

The letter said that “the jury wonders and some believe that she had other options available to subdue Mr. Crutcher,” but that jurors were unsure whether Officer Shelby had training that would have allowed for a different course of action. “The jury could not, beyond a reasonable doubt, conclude that she did anything outside of her duties and training as a police officer in that situation,” the letter said.

The foreman said the law, as written, prevented the jurors from returning a guilty verdict, regardless of the troubling video. “I believe that I speak for the whole of the jury, when I say that the general public in these types of cases are unaware of just how specifically the rule of law dictates how a jury must reach a verdict,” the foreman wrote.

The rise of video evidence has also raised new questions about how it should be properly presented to juries.

The Milwaukee case this month pivoted largely on body camera video showing the 1.69 seconds between two shots that Officer Heaggan-Brown fired at Mr. Smith.

At the time of the first shot, Mr. Smith was holding a gun. When the second bullet was fired, the gun had been tossed over a fence. The prosecution gave jurors a frame-by-frame view of the episode, relying on the premise that the officer had committed a crime when firing the second shot.

Steven Kohn, a lawyer for Mr. Heaggan-Brown, said the prosecution had tried to slow down what had actually taken place in a split second.

“My takeaway from this is that if either side is focusing their case on slow-motion and frame-by-frame deciphering of what occurred, it is a misuse of video,” Mr. Kohn said.

For all the debate over how videos should be used at trial, jurors say such evidence undeniably has value. In one recent high-profile case in Baltimore, where there was cellphone and surveillance video but no body camera footage, a juror said she wished for the clarity that more video might have provided. Jurors there in 2015 failed to reach verdicts on any of the counts against Officer William G. Porter, one of six officers charged in connection with the in-custody death of Freddie Gray. The judge declared a mistrial, and prosecutors later dropped the charges.

Susan Elgin said she had been the only juror who wanted a full acquittal of Officer Porter. Had there been more video footage, Ms. Elgin said in an interview, “it would probably have made all the difference, whether he was even charged.”

Still, videos have not delivered the outcomes some prosecutors have hoped for. Last week, Seth Tieger, a prosecutor in the Cincinnati case, described video as “the ultimate witness” in the retrial of Raymond M. Tensing, a former University of Cincinnati police officer charged with murder and voluntary manslaughter for fatally shooting Samuel DuBose, an unarmed motorist, in 2015.

“That video has no bias,” Mr. Tieger said during closing arguments. “This isn’t 20/20 hindsight. It’s a view of what happened.”

But on Friday afternoon, jurors told a judge in the case that they simply could not reach agreement on what had happened. They were split almost evenly on the case against Mr. Tensing, according to a note they wrote to the judge, who announced another mistrial.

Jess Bidgood and Christina Capecchi contributed reporting for this story.

A version of this article appears in print on , Section A, Page 10 of the New York edition with the headline: Videos of Police Shootings Give Few Easy Answers. Order Reprints | Today’s Paper | Subscribe